An ionographic print head, as represented in FIG. 4, consists of a high-frequency wiring arrangement (HF) located on the top of a dielectric plate. This wiring arrangement must be matched to a hole structure present in the underside of the said dielectric plate in an initial electrode system, referred to as a finger electrode system. A further, second plane of electrodes, provided with a hole structure, referred to as a dot matrix electrode, is maintained at a distance of some 200 .mu.m from the finger electrode system by means of a dielectric spacer, likewise provided with holes, or a separation layer. Due to the fact that the hole structures of the individual planes are aligned precisely above one another, a hole system is created beneath the dielectric plate of the HF wiring system, in which a plasma can be ignited by means of a coupled HF current. In the plasma there occur, inter alia, negative charges, which are accelerated by means of a more positive potential imposed at the dot matrix electrode. The accelerated charges penetrate the hole structure at the end of the dot matrix electrode (at the end of the acceleration path) impinge on a rotating drum, and are stored there. A latent point charging pattern pertains, which is then applied in the conventional manner by means of toner onto paper or plastic film and burned in, as described in U.S. Pat. No. 4,891,656.
The dielectric plate is usually made of Muscovite mica (potash mica, H.sub.2 KAl.sub.3 (SiO.sub.4).sub.3) ; see U.S. Pat. No. 5,030,975; U.S. Pat. No. 4,628,227; U.S. Pat. No. 4,958,172; and is bonded with the HF electrodes on the basis of an epoxy adhesive capable of being hardened by UV radiation. In view of the fact that mica breaks very easily, mechanical shocks and the slightest flexure or rotation of this layer system is to be avoided. In instances in which flexure cannot be avoided, use must be made of a flexible dielectric plate. For instance, a silicone plastic can be used, which is capable of being hardened by UV radiation, and which can be applied in silk screen printing processes. In addition to the sensitivity to fracture of mica, the resistance to plasma discharge of both the mica and the silicone plastic used as an alternative, is low. As a result, the ionographic printing head only has a short service life. A plasma which is ignited in the atmosphere delivers, during ion production, ozone and nitric acid as byproducts, which corrode the mica; the silicone plastic used as an alternative is subject to erosion. Therefore, both are subject to damage caused by the plasma. A further disadvantage in the use of mica derives from the fact that it is only obtainable in small dimensions, with the result that only small-format print heads can be obtained.
The plane which follows the dielectric plate in the construction of the head is a perforated finger electrode system, usually made of stainless steel or molybdenum, which (see U.S. Pat. No. 5,030,975) as secured to the underside of the dielectric plate by means of an adhesive which hardens. The next plane to follow is a plane provided with slots, which serves as the spacer between the finger electrodes and the screening electrode system. This dielectric spacer, about 200 .mu.m thick, consists either of UV-hardening plastic, which is applied by a silk screen printing process; or of photolithographic film, capable of texture structuring, such as VACREL from DuPont (see U.S. Pat. No. 4,745,421, and U.S. Pat. No. 4,890,123). Finally, the dot matrix electrode, likewise provided with a hole structure, is bonded to the spacer element by means of a silicone adhesive. The superimposed hole/slot/hole structures of dot matrix electrode/spacer/finger electrode define a system of small hollow cavities with a volume of about 6.times.10.sup.6 .mu.m.sup.3, in which a plasma can be ignited via a wiring arrangement located above the cavities.
To date, the ionographic printing technique has not yet succeeded in achieving an economic breakthrough because of two basic problems; namely, short service life and excessive fluctuations of the charge stored per image element, have stood in the way of the advance of this technique. The excessively short service life is, as already mentioned, attributable to the plasma erosion of the polymers used. The second basic problem, the severe fluctuation of the charge stored per image element, can be attributed to the excessive deviation in the layer thicknesses (mica, spacers, adhesive layers, etc.) and the orientation of the HF wiring to the plasma cavity. In addition to this, the dimensions of the many small plasma cavities are also subject to considerable fluctuation, caused by the manufacturing technique (screen printing, lamination, etc.).
In order to counteract the plasma erosion, the mica layer has been replaced in the past by glass, ceramics, or glass ceramics. However, the high sintering temperature which occurs during the manufacture of the layer has proved to be an impediment to such replacements; see U.S. Pat. No. 4,958,172. Likewise, the use of porcelain-coated steel sheets has again been rejected because of their uneven surface.
The second basic problem is the lack of sufficient grey tone gradation, caused by the sharp charge fluctuations between the individual image dots. For coloured or black-white quality prints on paper or plastic film, at least 64 grey gradations are required.
From this is derived a maximum charge fluctuation of .DELTA.Q=approx. 5%. This requirement in turn demands the finest possible manufacturing tolerances in fabricating the printing head. Based on .DELTA.Q.ltoreq.5%, a value of .+-.2 .mu.m is derived for the displacement (see FIG. 3) between the HF wiring plane and the finger electrode plane, while a value of .+-.23 .mu.m is permissible for the displacement between the finger electrode plane and the dot matrix electrode plane. Likewise, the thickness tolerance of the dielectric spacer may only amount to .DELTA.d.+-.2 .mu.m. For the diameter deviation of the holes in the finger electrode, a value is permitted of .+-.2.6 .mu.m for a hole diameter of 125 .mu.m, and for the dot matrix electrode a deviation of .+-.1 .mu.m for a hole diameter of 163 .mu.m.
Therefore, an object of this invention is to create a precisely made ion generator with a longer service life.